LONDON (Reuters) - Lewis Hamilton declared his new Mercedes to be the big sister of last year's temperamental title winner but Formula One's first pre-season test of 2018 still left plenty of room for conjecture.

The four-times world champion, who can expect to start the campaign in Australia on March 25 as a favourite, said bad weather, a resurfaced and smoother track and heavier cars had made comparisons tricky.

Hamilton was fastest over the opening four days at Barcelona's Circuit de Catalunya with a lap of one minute 19.333 seconds on Thursday, after snow and rain blew out Wednesday's track action.

The time was quicker already than anything the Briton did in 2017 pre-season testing, when Mercedes were behind only Ferrari, and not far off the 1:19.301 that team mate Valtteri Bottas produced on day six of last year.

And he did it on the medium Pirelli tyres, slower than the ones used by Ferrari -- whose best lap was a 1:19.673 by Sebastian Vettel on softs on Tuesday. But plunging track temperatures made it hard to get heat into the tyres.

"The track is quicker than it was in the past, much smoother. A lot more grip," offered Hamilton by way of explanation.

"It’s difficult to say whether the car's that much better or how much of that is the track and how much is the grip. And how much of that is the tyres.

"So over the next days, I guess next week, we’ll get a better understanding."

The second, and final, four-day test in Barcelona starts next Tuesday.

Hamilton felt the new W09 seemed a much better car than last year's 'diva', a nickname bestowed on it early on by team boss Toto Wolff for its sometimes twitchy and temperamental behaviour.

If he felt ready to race in Melbourne, he recognised there was still plenty of work to lay the "building blocks for a title fight".

"But we have no idea where people are," he added. "It looks like Red Bull’s quite quick, looks like the Ferrari’s quite quick but no-one really knows anything."

One certainty is that the cars are heavier to handle as a result of the new halo head protection device, which adds at least another seven kilograms.

However, teams test with a variety of fuel loads and settings, which makes a comparison of the times fairly meaningless.

Other take-aways from the track were that Honda had moved on from their failed partnership with McLaren and made a big step up in reliability after switching to Toro Rosso, who did more laps than anyone.

McLaren appeared to be settling in well with new engine provider Renault, whose works team has been tipped by Wolff to make the biggest step up of any this year. Force India completed the fewest laps, just behind Haas.